A few hours after Greenpeace activists invaded a business meeting in Durban on Monday, the organisation's international director, Kumi Naidoo, went to the Durban central prison where some of them had been taken. It was a painful case of deja vu, he says. They were being held in the very same cells that he and other anti-apartheid activists had so often been chucked in the 1980s.

Naidoo is a Durban man. The city and the long South African struggle against apartheid in the 1970s and 80s shaped him, and his return last week for the UN climate talks as one of the world's most influential environmental activists brings back the memories.

The route that 20,000 people on a civil society march took at the weekend included several places where he had been arrested and beaten up for opposing apartheid. He had been at school with two of the policemen who came to the arrest the Greenpeace activists; and South Beach, where we talked, was where his father used to bring him to show him where the white kids and no one else were allowed to play.

"The first time I heard about Robben island [where Nelson Mandela and other African National Congress leaders were imprisoned] was when I was about 10. My brother asked Dad why we could not play on the beach. He said 'don't ask. You'll end up on Robben island'. We kept asking each other what it was."

Naidoo's family is Indian working class. They had been forcibly resettled with thousands of others to Chatsworth township, about 30 miles out of the Durban, and he grew up with a picture of Gandhi on the wall, even though the Indian liberation activist had left Durban before he was born.

The Soweto uprising of 1976, anti-apartheid activist Steve Biko's murder in prison and, especially, the student uprising in 1979, all developed his radicalism, he says. He was twice expelled from school for organising opposition to apartheid, but each time challenged the government in the courts and was reinstated.

"We had very conservative teachers. Once I was asked to write an essay about the two people I admired the most. I said Nelson Mandela and Gandhi. My teacher said: 'You want me to get killed? You want me to mark this radical shit?' "

"In one sense I am grateful to the apartheid regime. It made me a full-time activist. My brother and I made a pact that we would give our lives to the struggle."

Naidoo went on to run a children's home, and became a community organiser, where he was arrested several times and charged for violating the state of emergency, and civil disobedience. He ducked underground, worked in Zimbabwe and then spent four years in exile in England.

"I considered very much joining the ANC armed struggle. Apartheid in itself was a system of violence. But my first influence was Gandhi. I was more interested in the politics. It was a very emotionally traumatic time. I was a teenager. I fully understood the choices people made. You have to remember the ANC was a religion in those days."

The 1994 elections changed everything. "Suddenly we had friends who were cabinet ministers, or generals. But I felt we had to evolve a new way of addressing power. We came up with the idea of 'critical solidarity'. I founded the South African National NGO coalition but I began to realise that even if we had the cleanest politicians, the most anti-corrupt leaders, the whole nature of power had changed so much that there was only so much that we could do.

Naidoo then spent 10 years "on the road", leading the Make Poverty History campaign and other civil society groups. "Real power was shifting to the global level. Even with the best president there is no way he would make progress on things like environment and trade, even things like HIV/Aids," he says.

He has been criticised for not being an environmentalist, but he responds that the struggle for human and climate justice is similar to that against apartheid. Apartheid was a system of differentiation and injustice maintained by the powerful in the same way as governments and industries abuse nature.

"Apartheid affected one country but challenged the world. This is about the future. The bunch of adults leading us today are sleepwalkers, saying one thing about climate change yet doing nothing.

"For some, like the Turkana people in northern Kenya, the tipping point has already come. It is so unfair that the poor will pay for climate change with their lives.

"What I see now is very similar to the moment of change from apartheid to democracy. I think there was a moment in 1988 when you felt that the writing was on the wall for apartheid. The system was crumbling. I am hoping that sanity will now prevail with climate change and the environment, just as it did with apartheid. It may be naive optimism, but I believe we are at that point now. We are seeing the last kicks of the climate deniers' horse."

Returning to Durban hurts, though. "We have had 15 years of democracy and there are still 20 million people here in poverty. What hurts me most is that our government keeps talking about concern about climate change, yet 2.5 million people do not even have electricity and the government is building two of the largest coal-fired power stations in the world."

He says his daughter convinced him to go for the Greenpeace job. "She was in London and had seen me on the BBC. I was in the 19th day of a hunger strike and looking like a skeleton. She said: 'Dad, go for it. Greenpeace is about the future. It talks and it acts.' She even helped me fill in the form."